Stainless steels are utilized extensively in a number of applications in the chemical, petrochemical, and energy fields, and their usage in these applications is continuing to increase. Stainless steel castings have been widely used in machines and equipment for pollution control, nuclear energy installations, refinery equipment, coke sintering, electrical power apparatus, chemical plants, food processing, marine-related equipment, and others.
Stainless steels also are used in some recreational products such as heads for golf club irons. Golf irons include club numbers 1 through 9, pitching and sand wedges, putters, chippers, and the like. Many of the stainless steel golf club heads are produced as investment castings to achieve intricate design configurations for improved playing performance. Stainless steel alloys used in investment-cast golf club heads usually are martensitic types, such as type 431, for example, or martensitic precipitation-hardenable grades, such as 17-4 PH, for example. Investment cast type 431 stainless steel golf club heads have a hardness of typically Rockwell C36-40 in the as-cast condition, and have a hardness of about Rockwell C23-25 after heat treatment for one hour at 1550.degree. F., followed by slow cooling, then reheating for 1 to 11/2 hours at 1250.degree. F. These heat treatments produce essentially the minimum hardness that I have been able to achieve in investment cast type 431 golf club heads. Investment cast type 17-4 PH stainless steel golf club heads have a typical hardness of Rockwell C40-46 in the as-cast condition, and about Rockwell C32-35 after heat treatment for two hours at 1125.degree.-1150.degree. F., followed by air cooling. This heat treatment produces about the minimum hardness that I have been able to achieve in investment cast type 17-4 PH golf club heads.
Some stainless steel golf club heads, which are produced in either cast or forged form, are made from all-austenitic materials, such as an 18-8 or similar types, which have a low yield strength and hence tend to become bent by plastic deformation in play. Additionally, the austenitic materials are more highly alloyed and hence are more expensive to produce than golf club heads of the 431 and 17-4 PH types.
Some golfers have a preference for the "feel" or playing characteristics of forged carbon steels, such as a medium carbon AISI 1035 steel. These forged carbon steel golf club heads typically have a hardness of about Rockwell B90, although hardnesses as low as Rockwell B82-85 have been reported.